GeoLog

Zagros Mountains

Imaggeo on Mondays: What a thin section has to say about the deformation of the Zagros Mountains

Imaggeo on Mondays: What a thin section has to say about the deformation of the Zagros Mountains

The impressive Zagros Orogeny, as seen from a bird’s-eye view, has featured on Imaggeo on Monday’s blog posts a few times recently. From its fluvial dissection features, through to a false colour LANDSAT 7 image which reveals a velociraptor hiding among fold and thrusts, we’ve looked at the broad scale structures which shape the Zagros mountains. This week, the scale changes entirely: we zoom right into the fabric of the Zagros Mountains rocks, as shown by the minerals in a thin section. Despite the close-up and small-scale view thin sections offer, they can still reveal huge amounts of information about the past history of rocks.

The thin section image above, taken by Amirhossein Mojtahedzadeh, a member of the Geological Society of Iran, is from the northwestern part of the Zagros mountain belt, which spans Kurdistan (in northwester Iran). Here, the mountains have suffered multiple stages of metamorphism and deformation, as evidenced by the large twinned plagioclase (the stripped white and black mineral which dominates the image) in the centre of the view. Plagiocalse is a common rock-forming mineral of the silicate family. Amongst their many properties, plagioclase crystals commonly form twins, which essentially means that a single crystal of a mineral has two or more parts in which the crystal lattice is differently orientated (which is explained in full detail in this great blog post by former EGU network bloggers, Between a Rock and a Hard Place). This property of plagioclases is an important tool in petrological analysis as it usually results from a change in the conditions while the mineral is forming, hinting at larger scale changes in the rock‘s environment, such as an increase in the local environments pressure or temperature, for example.

The rock in the thin section above, is a metamorphosed olivine gabbro – typically formed in igneous environments – except that the plagioclases reveal tell-tale signs that this particular specimen has been reheated or buried, perhaps even both, after if formed. Local strain in the rocks results in the twinned crystals having a lenticular shape. Plastic deformation (meaning the crystal will never go back to the shape it was originally) driven by situations of intense temperature and pressure, causes atomic layers in minerals to slide past each other without friction and thus changes the orientation of twins. Whilst studying the rock, Amirhossein found that sometimes these conditions made elongated aggregates and led to the minerals gaining a new crystal structure.

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.

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Edit (26/10/2015): Following comments by Graham Cunningham on Facebook, this text was improved by substituting silicon family to silicate family in the sentence: Plagiocalse is a common rock-forming mineral of the silicate family.

 

Imaggeo on Mondays: Velociraptor in the Zagros Mountains

A velociraptor in the Zagros fold and thrust belt. Credit: Stephane Dominguez (distributed via  imaggeo.egu.eu)

A velociraptor in the Zagros fold and thrust belt. Credit: Stephane Dominguez (distributed via imaggeo.egu.eu)

How many times have you turned your head up to the sky and spotted familiar shapes in the clouds? Viewing structures from afar can reveal interesting, common and, sometimes, funny patterns.

Satellite images are often used to map geological terrains. They offer a bird’s eye view of the planet and the opportunity to see broad scale structures, the scale of which would be impossible to grasp from the ground. They can, from time to time, much like when you cloud spot, reveal interesting and unexpected features too!

The image above is a processed LANDSAT 7 Satellite image. Stephane Dominguez, a researcher at the University of Montpellier, acquired the image to study the Zagros Fold and thrust belt: the result of the collision of the Iranian Plate and the Arabian Plate.

Whilst studying the image, Stephane noticed an uncanny resemblance…who knew the Zagros mountain belt hosts a velociraptor? Stephane modified the image, using Photoshop, to obtain the false colours which highlight the dinosaur shape in the image. A little thumbnail of a velociraptor is included too, for comparison!

The surface morphology of the image is dominated by EW trending folds, with partially eroded cores. Darker/black areas correspond to salt diapirs that reached the surface in the fold cores or along reverse faults bounding the folds. We’ve featured the Zagros Mountains in our Imaggeo on Monday’s posts recently; you can find more details on the geology of the region here.

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.

Imaggeo on Mondays: Scales of fluvial dissection

Imaggeo on Mondays: Scales of fluvial dissection

High peaks, winding river channels and a barren landscape all feature in today’s Imaggeo on Mondays image, brought to you by Katja Laute, a geomorphologist from Norway. 

This photo was taken from an airplane flying over the Zagros Mountains in Iran. The Zagros Mountain range stretches south and west from the borders of Turkey and Russia to the Persian Gulf, and is Iran’s largest mountain range. The mountain range has a total length of 1500 km and stretches from north eastern Iraq, to the Strait of Hormuz. Many peaks are higher than 2900 m. The tallest mountain is Zard-Kuh at an elevation of 4548 m.

The Zagros fold and thrust belt was formed by the collision of the two tectonic plates- the Iranian Plate and the Arabian Plate. The collision resulted in parallel folds, which are seen as broad anticlines forming the high mountain peaks, , and orogenically are the same age as the Alps. The Zagros Mountains are made up primarily of limestone and dolomite – a sedimentary carbonate rock primarily composed of the anhydrous carbonate mineral of the same name.

The region exemplifies the continental variation of the Mediterranean climate pattern, with a snowy, cold winter and mild rainy spring followed by a dry summer and autumn. In winter, low temperatures can often drop below – 25 °C and many mountain peaks exhibit snow even in summer. The most common ecosystems in the Zagros Mountains are the forest and steppe areas which have a semi-arid temperate climate.

The photo gives an amazing impression of different scales of fluvial dissection. The landscape consists of valleys and their included channels organized into a connecting system known as a drainage network. The powder snow enhances nicely the dendritic drainage pattern.

 

By Katja Laute, Geomorphologist, Trondheim, Norway

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.